A light-emitting element includes a layer containing a light-emitting material (a light-emitting layer) between a pair of electrodes (an anode and a cathode). It has been reported that a variety of organic compounds can be used as the light-emitting material.
It is said that light emission mechanism of a light-emitting element is as follows: when voltage is applied between a pair of electrodes with a light-emitting layer interposed therebetween, electrons injected from a cathode and holes injected from an anode are recombined in the light emission center of the light-emitting layer to form molecular excitons, and energy is released to emit light when the molecular excitons relax to a ground state. A singlet excited state and a triplet excited state are known as excited states, and it is thought that light emission can be obtained through either of the excited states.
In such a light-emitting element, since more excitons are generated in a triplet excited state than in a singlet excited state, emission efficiency of the light-emitting element can be increased by using a material that can emit light from a triplet excited state (a phosphorescent material). Therefore, a phosphorescent material has been attempted to be used as a light-emitting material a large number of times.
A metal complex where iridium (Ir) is the central metal (hereinafter, referred to as an Ir complex) is a typical phosphorescent material which emits green to blue light (for example, see Patent Document 1). Disclosed in Patent Document 1 is an Ir complex where a triazole derivative is a ligand.